Conventional programmable logic controllers (PLC) are prevalent in various industries since they can provide a means for intelligently controlling, among other things, mechanical and electrical processes. Consistency and reliability of specific types of PLCs affects their use within process control applications. It is common for known PLCs to be sufficiently functional for a variety of uses, including traffic control, production and assembly lines, and electromechanical machinery control. However, PLCs have not been deemed suitable for use in railroad signal systems based in part upon the non-vital nature of known PLCs.
Railroad grade crossings often involve motor vehicle traffic that cross railroad tracks, the situs of which is notorious for motor vehicle-train collisions. A variety of warning systems intended to warn vehicle operators of approaching trains have employed two major warning systems. These major warning systems include an audible signal sent from the train itself and a visual warning signal located at the site of the grade crossing. The visual warning system almost always includes passive markings (road signs, roadway painted markings, etc.), but active markings (drop down gates, flashing lights, etc.) are not always employed.
Visual railroad signaling device functionality is often governed by national and/or local governing body signaling standards. By example, within the United States, any device designed for railroad signal service must conform to established federal, state and railroad signal standards for design and operation of the signaling devices. It is often the case that an audible signal and/or passive warning methods are not sufficient to provide a motor vehicle operator with sufficient time to avoid a collision. In the case of those crossings that do not have an active vital and preemptive visual warning system, the likelihood of a collision is increased significantly. It is therefore advantageous to provide an active vital and preemptive visual warning system. However, it is cost prohibitive for every grade crossing to have an active vital and preemptive warning system that adheres to the local signaling standards. It is advantageous to provide a cost effective active vital and preemptive warning system.
Railroad signal standard practice for the design and function of signal systems is based upon the concept of a vital system. A vital system is often characterized as being failsafe and consistent with the closed circuit principle. A signal design is failsafe if the failure of any element of the system causes the system to revert to its safest condition. Operation at the safest condition is often activation of the warning system. In the case of railroad signal systems, failsafe design requires that if any element of the active system cannot perform its intended function that the active crossing warning devices will operate and continue to operate until the failure is repaired. In the case of railroad wayside signal systems, failsafe design requires that if any element necessary to the safe and proper operation of the system cannot perform its intended function that the system will revert to the safest condition, i.e. a red signal indicating stop or proceed at restricted speed according to rules is in effect. A signal design is in conformance with the closed circuit principle when the components of the system do not share elements which could afford alternative energy or logic paths, as these elements would violate the failsafe principle. It would be highly advantageous to employ cost effective and failsafe vehicle detection systems using microprocessors or PLCs.